The provision of an electronic tape rule in which a digital readout of tape position is combined with an ability to perform calculation using tape position as an input is described in Japanese Utility Model No. SHO 51-71937, U.S. Pat. Nos. 4161781 (Hildebrandt), 4181959 (Tateishi), 4181960 (Tateishi et al.), 4316081 (Washizuka) and UK Patent Specifications Nos. 2102121 (Souic Tapei) and 2052745 (Wines). However the above references require the user to carry out two key operations to display the result of a calculation based on a previous tape position and do not provide for updating the result continuously based on current tape position.